1. Field of the Invention
The present invention relates to a mobile communication method, a mobile communication system, a router, and a program; and more particularly to a mobile communication control method, a mobile communication system, a router, and a program that employs IP (Internet Protocol) technology.
2. Description of the Related Art
With the development of mobile communication technology in recent years, the standardization of Mobile IPv6 has advanced in the IETF (Internet Engineering Task Force).
FIG. 1 shows an example of the configuration of a mobile communication system of the prior art according to Mobile IPv6.
As shown in FIG. 1, in the present example of the prior art, two home agents 130a and 130b are provided, each home agent having a home core network 131a and 131b, respectively. Communication is realized by means of the transmission and reception of IP packets between mobile terminal 110a, which contracts with the communication provider that has established home agent 130a and for which movement information is managed by home agent 130a, and mobile terminal 110b, which contracts with the communication provider that has established home agent 130b and for which movement information is managed by home agent 130b. Mobile terminal 110a is present within access network 121a that is under the jurisdiction of access router 120a. Mobile terminal 110b is present within access network 121b that is under the jurisdiction of access router 120b. Mobile terminals 110a and 110b each have home addresses (HoA) that are their own IP addresses, and these home addresses (HoA) are registered in home agents 130a and 130b, respectively.
The following explanation regards the method of communication between two mobile terminals 110a and 110b in the mobile communication system shown in FIG. 1.
Explanation first regards the position registration process of mobile terminals 110a and 110b. 
When mobile terminal 110a moves from home core network 131a, which is under the jurisdiction of home agent 130a established by the communication provider with which mobile terminal 110a is contracted, or from another access network to access network 121a that is under the jurisdiction of access router 120a, the prefix of the IP address of access router 120a and the MAC address of mobile terminal 110a are used in mobile terminal 110a to generate the care-of Address (CoA) which serves as the IP address of the movement destination. Then, a position registration request (BU: Binding Update) is carried out by sending the combination of the home address of mobile terminal 110a and this care-of address to home agent 130a. A position registration request (BU) is also carried out in the same way in mobile terminal 110b for home agent 130b by using the home address of mobile terminal 110b and the care-of address that has been generated by access network 121b. 
In home agent 130a that has received the position registration request (BU) from mobile terminal 110a, the validity of the request is first confirmed, following which the position registration request (BU) is registered in the binding cache (BC) in home agent 130a in association with the home address and care-of address that have been transmitted from mobile terminal 110a and a position registration response (BA: Binding Acknowledgement) that indicates that the position registration has been carried is transmitted to mobile terminal 110a. The position registration of mobile terminal 110b is also carried out in the same way in home agent 130b, and a position registration response (BA) is transmitted to mobile terminal 110b. 
In this way, the current positions of mobile terminals 110a and 110b, which have contracted with the communication providers that have established home agents 130a and 130b, respectively, are each managed in home agents 130a and 130b respectively, whereby the movement information of mobile terminals 110a and 110b is managed.
Explanation next regards the process when communication is carried out by the transmission and reception of IP packets between mobile terminals 110a and 110b following implementation of the above-described position registration process, taking as an example a case in which IP packets are transmitted from mobile terminal 110a to mobile terminal 110b. In the following explanation, home addresses of mobile terminals 110a and 110b are “HoA1” and “HoA2,” respectively, and the care-of addresses of mobile terminals 110a and 110b are “CoA1” and “CoA2,” respectively.
When IP packets are transmitted from mobile terminal 110a to mobile terminal 110b, IP packets for which the origination address is “HoA1” and the destination address is “HoA2” are encapsulated in mobile terminal 110a with “CoA1” as the origination address and the IP address “HA1” of home agent 130a as the destination address, and these encapsulated IP packets are then transmitted from mobile terminal 110a. 
Since the destination address of the IP packets that are sent from mobile terminal 110a is the IP address “HA1” of home agent 130a, the IP packets are delivered to home agent 130a by way of access router 120a. Upon reception in home agent 130a of the IP packets that are transmitted from mobile terminal 110a, the encapsulation of the received IP packets is removed: The IP packets for which the origination address is “HoA1” and the destination address is “HoA2” are then transmitted.
The destination address of the IP packets that are transmitted from home agent 130a is “HoA2” and the IP packets are therefore delivered to home agent 130b. Upon receiving the IP packets that have been transmitted from home agent 130a in home agent 130b, the binding cache (BC) in home agent 130b is consulted to find the care-of address “CoA2” of the mobile terminal 110b that is the destination of the IP packets. The received IP packets are then encapsulated with the IP address “HA2” of home agent 130b as the origination address and “CoA2” as the destination address, and the encapsulated IP packets are transmitted from home agent 130b. 
The destination address of the IP packets that are sent from home agent 130b is “CoA2”, and the IP packets are therefore delivered by way of access router 120b to mobile terminal 110b having the care-of address “CoA2.” Upon reception of the IP packets that have been sent from home agent 130b in mobile terminal 110b, the encapsulation of the received IP packets is removed, whereby the IP packets for which the origination address is “HoA1” and the destination address is “HoA2” are received.
In this way, mobile terminals 110a and 110b that have contracted with communication providers that have established home agents 130a and 130b each manage their current positions in home agents 130a and 130b in mobile IPv6. In the event of movement of mobile terminals 110a and 110b, movement transparency is ensured by the transfer of IP packets addressed to mobile terminals 110a and 110b to mobile terminals 110a and 110b by home agents 130a and 130b. This technology is disclosed in, for example, JP-A-2005-26941.
The afore-mentioned mobile IPv6 includes a mode for optimizing the communication path between mobile terminals 110a and 110b that transmit and receive IP packets. The following explanation regards the process when optimizing a communication path.
When IP packets that have been transmitted in from mobile terminal 110a are encapsulated in mobile terminal 110b with the IP address “HA2” of home agent 130b as the origination address and “CoA2” as the destination address, the combination of the home address “HoA2” of mobile terminal 110b and the care-of address “CoA2” is transmitted to mobile terminal 110a and the position registration request (BU) implemented in mobile terminal 110b. 
In mobile terminal 110a that receives the position registration request (BU) from mobile terminal 110b, the validity of the request is first verified, following which the combination of the home address “HoA2” and care-of address “CoA2” of mobile terminal 110b is registered in route optimization memory and a position registration response (BA) then transmitted to mobile terminal 110b. 
When IP packets addressed to mobile terminal 110b are subsequently transmitted from mobile terminal 110a, IP packets are transmitted in mobile terminal 110a with the transmission origin address as “CoA1,” the destination address as “CoA2,” and “HoA1” added as the home address option. These IP packets are delivered to mobile terminal 110b without passing by way of home agents 130a and 130b. 
This optimization of the communication path avoids the occurrence of delay that is produced when a communication path becomes redundant when transmitting and receiving IP packets between mobile terminals 110a and 110b. This type of technology is disclosed in, for example, JP-A-2005-33469.
In this type of method of optimizing the communication path between mobile terminals 110a and 110b, however, care-of addresses “CoA1” and “CoA2” that indicate the current positions of mobile terminals 110a and 110b are reported to the communication partner, whereby one's current location is revealed to the communication partner.
In communication in which the communication path is optimized, an edge mobility technique can be considered in which IP packets that are transmitted and received between mobile terminals 110a and 110b are encapsulated or decapsulated by access routers 120a and 120b. In this edge mobility technique, the above-described position registration requests (BU) to home agents 130a and 130b, the generation of care-of addresses of mobile terminals 110a and 110b, and the path optimization process are carried out by access routers 120a and 120b in the place of mobile terminals 110a and
As a result, the care-of addresses “CoA1” and “CoA2” of mobile terminals 110a and 110b are managed not by mobile terminals 110a and 110b but by access routers 120a and 120b, and in addition, when mobile terminal 110a and mobile terminal 110b transmit and receive IP packets by the above-described optimized path without passing by way of home agents 130a and 130b, the care-of addresses “CoA1” and “CoA2” of mobile terminals 110a and 110b are transmitted and received only between access routers 120a and 120b and the care-of addresses are not reported to the communication partner, whereby the revelation of one's current location to a communication partner can be avoided.
When optimizing a path to realize communication as previously described, compared to a case in which communication is realized by way of home agents, not only must position registration requests (BU) and position registration responses (BA) to these position registration requests be transmitted and received between mobile terminals and home agents in the control system, but these items of information must further be transmitted and received between the mobile terminals. In addition, return routability (RFC 3775) must be mounted for protecting position registration requests (BU) and position registration responses (BA) between mobile terminals. In addition, when communication is realized in the data system between mobile terminals, the need arises to give a home address option of each of the mobile terminals to the IP packets that are transmitted and received, thereby increasing the packet size of IP packets that are transmitted and received.
As a result, when optimizing a path to realize communication, compared to a case in which communication is realized by way of home agents, despite the avoidance of redundancy of communication path and the reduction of load relating to the routing process, an increase in the costs in the above-described control system and data system detract from any claim of optimization when viewed in terms of total cost.